The invention concerns a rib waveguide, a process for manufacturing it, its utilization as well as a light source containing this rib waveguide in accordance with the generic terms of the independent claims.
Frequency conversion of laser light by non-linear optical interactions has been accorded some attention since the sixties. Non-linear optical processes enable the generation of coherent laser light at optical frequencies (wavelengths), which cannot, or else only with difficulty, be generated by a direct laser process. In general, for such a frequency conversion process a laser is utilized as pumping source, the light beam of which propagates through a non-linear optical material. The non-linear optical interaction between the laser beam and the material leads to the effect, that a part of the pumped light is converted into light of a higher or lower frequency. Among the non-linear optical processes, the second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG) and optical parametric amplification (OPA) are of particular significance. These processes enable the generation of coherent laser radiation in the ultraviolet, visible, nearxe2x80x94and intermediate infrared spectral range, i.e., between 0.1 xcexcm and 10 xcexcm wavelength. Lasers which emit light at these wavelengths find applications in spectroscopy, optical data storage, medicine, biology, etc.
A further important non-linear optical process is the electro-optical modulation of laser light. Hereby an electrical field is applied to the non-linear optical crystal and with this the intensity or propagation velocity of the laser light is influenced. This effect can be utilized for transferring electronic signals to the optical beam. This makes possible the transmission of information with the help of optical systems, which today is finding widespread use in communications technology. Apart from this, the electro-optical effect is exploited for various other applications, such as in optical switches or in Q-switches in lasers for the generation of very short laser pulses.
A great number of crystalline materials which are suitable for non-linear optical interactions have been investigated. Among these, especially the class of the ferro-electric oxides has found attention, e.g., potassium niobate (KNbO3), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), barium titanate (BaTiO3) and KTP (KTiOPO4). In general, crystals of these materials manifest great non-linear optical susceptibilities, a material characteristic, which is a necessary prerequisite for efficient frequency conversion and electro-optical modulation. In particular potassium niobate on the basis of its outstandingly good characteristics has proved to be an excellent material for non-linear optical applications.
A further class of materials which has interesting non-linear optical characteristics are crystals based on borate compounds, such as xcex2-BaB2O4 (BBO), LiB3O5 (LBO), CsB3O5 (CBO) and CsLiB6O10 (CLBO). This group of non-linear optical crystals is distinguished by the fact, that its optical transparency in contrast to most of the ferro-electric oxides reaches far into the ultraviolet spectral range. On the basis of this characteristic, borate crystals are interesting for frequency conversions, in which ultraviolet laser radiation is generated.
The degree of conversion in the case of a non-linear process increases with the length of interaction of pumped beam and crystal and with the intensity of the pumped beam. In order to achieve a high degree of conversion, therefore frequently additional measures have to be undertaken, in order to increase the intensity of the pumped beam, this in particular, if the laser is operated in the continuous wave (cw) mode. Investigated as such measures were above all resonant processes in optical cavities and conversion processes in optical waveguides. Resonant conversion processes, in the case of which the non-linear optical crystal is placed in a cavity, provide the possibility of achieving very high degrees of conversion. However, they have the disadvantage, that they are very sensitive with respect to the mechanical adjustment of the optical components and to small fluctuations of the pumped beam wavelength. Therefore they normally require a complicated active feedback system for stabilization. In contrast, the utilization of waveguides for frequency conversion has the advantage of, solely on the basis of the very small cross sectional surface area of the waveguide and of the lateral guiding of the laser beam, assuring a high intensity over a long interaction length and of thus achieving a high degree of conversion. For this, already a single pass of the pumped beam through the waveguide is sufficient, which substantially reduces the demands of the mechanical and frequency-related stability in comparison with optical cavities.
Waveguides also offer advantages for electro-optical applications compared with volume crystals. By the restriction of the light to a very small surface over a long distance, the electrical voltage applied for the modulation can be kept very low, whereby the required electrical power is significantly reduced. Waveguides are in addition compatible with the fibreglass technology, which is utilized in today""s communications systems.
Of particular significance for applications are rib waveguides, i.e., channel-shaped waveguides, which guide the light in two directions and limit it to a very small surface.
The above explanations emphasize the significance of waveguides, in particular of rib waveguides, for non-linear optical applications. The manufacturing processes of such waveguides, however, are frequently technologically difficult and have to be adapted to the corresponding material characteristics. This invention is based on a process, which permits the manufacture of waveguides of good optical quality in non-linear optical crystals, while maintaining the advantageous characteristics of these materials.
For the manufacture of optical waveguides in non-linear optical crystals, different methods have been investigated, both chemicalxe2x80x94as well as physical ones. By means of ion diffusionxe2x80x94or ion exchange processes in the non-linear optical crystal, one, for example, succeeded in manufacturing waveguides in LiNbO3, LiTaO3 and KTP. Proving to be successful, e.g., was the diffusion or implantation of titanium (Ti) in LiNbO3. Within the Ti-doped zone, the refraction index is increased, while simultaneously the desired optical characteristics of the LiNbO3 are maintained. Almost all other non-linear optical crystals, however, are not accessible for this process, because the outside ion diffusion constants and the thermal stability are insufficient. Also the forced doping with the help of ion implantation does not achieve the objective, because the ion implantation of heavy ions such as Ti damages the host lattice structure through atom impacts and creates defects, so that no usable waveguides are produced. Among the physical methods, above all the implantation of light ions such as H+ or He+ have found applications. With these processes, the crystalline material is subjected to a bombardment of high-energy ions. This leads to the formation of a buried optical barrier, i.e., to a zone with a lowered refraction index, and to a wave-guiding layer underneath the surface of the crystal.
Fluck and others, in the publication xe2x80x9cLow-loss optical channel waveguides in KNbO3 by multiple energy ion implantationxe2x80x9d (J. Appl. Phys. 72 (5), 1671 (1992)) have demonstrated, that the manufacture of rib waveguides by ion implantation, e.g., into ferro-electric oxides such as KNbO3, is possible with a process, which uses several implantation steps. The rib waveguides manufactured in this manner, however, have the disadvantage, that they only conduct light of one polarization direction, while light with a polarization vertical to that does not propagate in the waveguide. In order, however, to make a frequency conversion in KNbO3 possible, the waveguide must necessarily also conduct light of both vertical as well as horizontal polarization. In the publication xe2x80x9cBlue-light second-harmonic generation in ion-implanted channel waveguides of new designxe2x80x9d (Appl. Phys. Lett. 69 (27), 4133 (1996)), Fluck and others have described a simpler process, which enables the manufacture of rib waveguides with only one implantation step. This process is the subject of the German patent application xe2x80x9cWellen-oder Streifenleiter, sowie Verfahren zu seiner Herstellungxe2x80x9d. Rib waveguides, which are manufactured with this process, have significantly improved optical characteristics in comparison with rib waveguides, which are manufactured using several implantation steps. On the basis of the narrow optical barrier on the side walls, however, they have a comparatively high attenuation.
In the patent document xe2x80x9cMethod for the fabrication of low loss crystalline silicon waveguides by dielectric implantationxe2x80x9d (U.S. Pat. No. 4,789,642) of Lorenzo and others, a process is described, which enables the manufacture of rib waveguides by means of ion implantation and etching. In doing so, a silicon substrate is exposed to the bombardment of high-energy oxygenxe2x80x94or nitrogen ions. These ions at a certain depth below the surface of the silicon substrate form a crystalline dielectric layer of silicon oxide (SiO2) or silicon nitride (Si3O4), which in comparison with the silicon layer above it manifests a significantly decreased refraction index. By etching or epitaxial growing of additional silicon in suitable places, subsequently ribs can be formed on the substrate surface and with this, rib waveguides. This process, however, is based on the particular material characteristics of silicon, where the implantation of heavy ions, such as oxygen or nitrogen effects the formation of a crystalline oxidexe2x80x94or nitride layer and with this a chemical change in the material. In contrast to this, the implantation of light ions, such as protons or helium into a ferro-electric oxide, e.g., KNbO3, or a borate, e.g., LBO, leads to the formation of a partially or completely amorphous damage zone due to atom impacts, not, however, to a chemical change of the material. This is also applicable for the implantation of heavy ions. The process of Lorenzo and others is therefore not transferrable to ferro-electric oxides or borates. In addition, this patent document refers exclusively to waveguides made of silicon for applications in the communication technology at light wavelengths of between 1.3 and 1.6 xcexcm. Because of the characteristics of silicon, these waveguides, however, cannot be utilized for frequency conversion processes.
It is the object of the invention to create a rib waveguide of good optical quality in a crystal, in particular a ferro-electric oxide or a borate. Furthermore, it is the object of the invention, to indicate a process, which enables the manufacture of such waveguides. Furthermore, it is the objective of the invention, to demonstrate the utilization of these rib waveguides for the efficient frequency conversion of semiconductor diode lasers and of solid-state lasers. In addition, it is the objective of the invention to create a light source, in which the light emitted by at least one primary light source is efficiently frequency converted.
The objective is achieved by the rib waveguide, the manufacturing process and the light source, as they are defined in the independent claims.
The rib waveguide in accordance with the invention is manufactured out of a crystal. It manifests a barrier layer in the crystal, the refraction index of which is lower than the refraction index of the crystal, and which limits the rib waveguide in the direction vertical to a certain surface of the crystal. In addition, it has a rib on the surface mentioned, the walls of which limit the rib waveguide in the direction parallel to the mentioned surface.
The process in accordance with the invention to a great extent circumvents the above mentioned difficulties in the manufacture of rib waveguides in non-linear optical crystals, such as, e.g., KNbO3. In essence, it consists of three process steps: (a) the implantation of high-energy light ions into a non-linear optical crystal, (b) the photo-lithographic production of a mask on the surface of this crystal or the utilization of a suitably pre-structured mask, e.g., made of silicon, and (c) the etching of the crystal from the surface, in order to form a rib screened by the utilization of the mask on this surface, whereby the lateral guidance of the light in the rib waveguide is assured.
The method of ion implantation exploits the permanent radiation damage, which is caused in the base material by the bombardment with ions. In the case of the utilization of light ions, e.g., H+, He+ or HE++, the radiation damage caused is concentrated at the end of the ion path. The zone close to the surface is in comparison only slightly damaged. The crystal volume in the depth as a result of the damage manifests a lower refraction index. In this way it is possible to guide light between the surface of the crystal and the damaged volume in the depth. Ion implantation provides the advantage of being able to manufacture tailor-made waveguides for very specific applications, thanks to the precisely controllable process parameters (ion dosage andxe2x80x94energy). The thickness of the wave-guiding layer (position of the optical barrier in the material) can be defined through the ion energy, while the refraction index in the optical barrier is defined by the ion dosage.
An important prerequisite for the usability of waveguides for non-linear optical processes consists in the fact, that the non-linear optical characteristics of the material are maintained during the manufacture of the waveguides. Precisely ion diffusionxe2x80x94or ion exchange processes can strongly reduce the non-linearity or destroy it completely. Here too, the ion implantation provides an advantage, because the damage to the material in the layer close to the surface, i.e., in the wave guiding zone, is comparatively minor and following the ion bombardment, it can be partially or even completely reversed.
The utilization of a photo-lithography process for the production of a mask for the subsequent manufacture of a rib waveguide, provides the benefit of being able to structure very precise patterns on the surface of the crystal. The photographic lacquer mask can thereupon be hardened using special chemical or physical processes, in order to make it more resistant to the subsequent etching process.
The utilization of a pre-structured mask has the advantage, that the mask structure, in particular the mask thickness, can be adapted to the subsequent etching process independent of the characteristics of the photographic lacquer. On offer as mask material is in preference silicon, also, however, wires or foils made of metal or glass can be used as mask.
The etching of the crystal from the surface and the rib on the surface of the crystal created in this manner leads to the manufacture of a rib waveguide, which manifests important advantages in comparison with the current processes. In particular, with this method the lateral guidance of the light is not achieved by an implantation process as in the case of the mentioned processes, which utilize multiple implantations or single implantation with inclined side walls. As a result, effects, which have an unfavourable influence on the waveguide, such as, e.g., tensions in the material, can be avoided. The etching process in addition can be adapted to the special characteristics of the material, whereby both wetxe2x80x94as well as dry etching processes can be utilized.
The rib waveguide in accordance with the invention can be used in optical frequency converters, electro-optical modulators and switches. In particular, the rib waveguide in accordance with the invention is utilized in combination with a diode laser or a solid-state laser for optical frequency multiplication, the generation of sum or difference frequencies and opto-parametric amplification or oscillation.
The rib waveguide in accordance with the invention is suitable for applications in optical frequency converters, electro-optical modulators as well as switches. For example, through the combination of a rib waveguide manufactured in KNbO3 and an AlGaAs or InGaAs diode laser, by optical frequency doubling light in the spectral range between 430 and 550 nm can be generated. As further possible pumped light sources, apart from semiconductor diode lasers also solid-state lasers are possible, above all Nd and Cr doped garnet, such as, e.g., YAG (Y3A15O12), GGG (Gd3Ga5O12), YSAG (Y3Sc2Al3O12), Gsag (Gd3Sc2Al3O12), GSGG (Gd3Sc2Ga3O12) as well as mYVO4, LiSAF and Ti:Al2O3. As a further example, through the combination of a diode laser, e.g., AlGaInP and a waveguide in a borate crystal, ultraviolet laser radiation in the wavelength range between 180 and 430 nm can be generated by frequency doubling. As further possible pumped light sources, apart from diode lasers also frequency-doubled solid-state lasers are possible. Borate crystals are suitable for non-linear optical processes of a higher order, e.g., the generation of thirdxe2x80x94or fourth harmonics, which, e.g., can be pumped with the mentioned solid-state lasers. Apart from this, with two light beams, which originate from two different laser sources, in a waveguide the generation of sumxe2x80x94or difference frequency can be achieved. As a further application, the waveguide can be utilized for the opto-parametric oscillation. These processes enable the generation of ultraviolet, visible or infrared light tuned to the wavelength, by pumping with a suitable laser. The rib waveguide is thereupon suitable for applications in communication technology, such as in electro-optical modulators, switches or directional couplers, whereby the waveguide is combined with suitable electrode structures. These components, for example, enable the optical coding of information onto a light beam or the switching of light between different glass fiber waveguides.
The light source in accordance with the invention contains at least one primary light source and at least one frequency converter, into which light emitted from the primary light source is coupled, whereby at least one frequency converter is the rib waveguide in accordance with the invention. Preferably suitable as primary light sources are semiconductor diode lasers (e.g., AlGaAs or InGaAs diode lasers), solid-state lasers (e.g., Nd:YAG, Nd:YO4 or Cr:LiSAF), wave guide lasers or fibre lasers. The light sources, which are based on the optical frequency multiplication (e.g., frequency doubling), opto-parametric amplification or opto-parametric oscillation, in preference contain a primary light source and a frequency converter. Light sources, which are based on optical sumxe2x80x94or difference frequency generation, in preference contain two primary light sources and a frequency converter.
The light emitted by the primary light source is preferably input to the rib waveguide directly or through an optical system. In order to keep the coupling losses at the first and second front side of the rib waveguide as low as possible, in preference both front sides of the rib waveguide should be provided with an anti-reflex coating. The residual light from the primary light source is uncoupled together with the light generated in the rib waveguide by the frequency conversion, e.g., light with the double frequency, through the second front side of the rib waveguide and in preference by means of an optical system, e.g., one or several lenses, bundled into a laser beam with a small divergence. The second front side of the rib waveguide can also be provided with a reflecting coating, so that the light coupled into the rib waveguide together with the light generated by the frequency conversion passes through the rib waveguide once again in the other direction and is then uncoupled through the first front side of the rib waveguide.
In the preferred embodiment, the crystal, (e.g., KNbO3) is bombarded with high-energy light ions (e.g., He+ ions with 2 MeV energy), in order to form a damaged zone buried underneath the surface with a lower refraction index. Subsequently, a mask made out of photographic lacquer is produced with the help of a photo-lithographic process, which covers narrow, strip-shaped zones. Finally the crystal is etched from above, whereby the zone covered by the mask remains unchanged. As a result, a rib is formed in this zone, which stands out from the surface of the crystal. Utilized for the etching is in particular a physical etching process (sputtering), for example, plasma etching with Ar+ ions.
In a second embodiment, following the process step (a) (implantation of a planar waveguide) the crystal is baked for a certain time period, in order to cure defects in the crystal lattice structure, which occur during the implantation. This curing process can also be carried out after the process step (c) (etching of a rib).
In a third embodiment, providing the non-linear optical crystal is ferro-electric, following the process step (a) (implantation of a planar waveguide) an external electric field is applied along the spontaneous polarization of the crystal, in order to reorientate ferro-electric domains created during the implantation and to pole the crystal completely along a designated direction.
In a fourth embodiment, following the process step (b) (photo-lithographic production of a mask) a process is utilized, which hardens the photo-lithographic mask and therefore makes it more resistant against the subsequent etching process. This process can consist of an additional implantation step with ions of low energy. As further possible measures for hardening the mask, chemical processes, baking or irradiation with ultraviolet light can be applied.
In a fifth embodiment, following the process step (c) (etching of a rib) a layer of low refraction material is applied to the surface of the crystal. This layer can in particular consist of oxidic compounds, such as SiO2, Al2O3, Ta2O5 or Nb2O5. This layer on the one hand serves to protect the rib waveguide from mechanical damage, on the other hand it has the purpose of reducing the optical diffusion losses at the interface waveguide-air.
In a sixth embodiment, the rib waveguide is split-up into two or more zones in such a manner, that light can be guided separately in each one of these zones. This is achieved by the implantation or further index barriers, whereby this takes place following the process step (a) (implantation of a planar waveguide) or (c) (etching of a rib).
In a seventh embodiment, following the process step (a) (implantation of a planar waveguide) a pre-structured mask is utilized instead of the mask made of photographic lacquer. This pre-structured mask in preference consists of silicon, however, wires, fibres or foils made of metal or glass can also be utilized.